Among the guilt that makes us so easily vulnerable as mothers is leaving the baby with someone else.

Over a pasta lunch the other day, a young friend said, ''If only people wouldn't say to me, 'You are going back to work NOW? But the baby is just three months old!' ''

My friend has an excellent job and an excellent day-care situation for the baby. She and her husband are trying to buy a farm on which to raise their family, and right now they need two incomes.

On the other side of the coin, a mother who doesn't go back to her job after her baby arrives may hear this:

''Well, WHEN are you going back to work? What a waste of all that education your parents worked so hard to provide . . . you'll feel trapped.''

Of course, in a Time magazine article March 9 entitled ''Backlash: The Undeclared War Against American Women,'' author Susan Faludi found that the rash of ''toxic day-care'' stories, which instilled guilt among working women by recounting the epidemic of abuse in day-care centers, masked the fact that the majority of child abuse goes on in the home.

Another ''mom guilt'' is the Caesarean Section, and a feeling that you are somewhat ''less'' of a mother if you've delivered your baby by means of a C Section.

I hear, ''I had to have a Caesarean Section. I didn't want one, but . . .''

And you find yourself explaining again.

For women who have had a Caesarean Section -- either at the last minute or the planned kind based on medical necessity -- there is a fine new book, ''Recovering From a C Section,'' (Longmeadow Press) by Margaret Blackstone with Dr. Tahira Homayun.

Ms. Blackstone writes about the lowered self-esteem women feel when they have this procedure and the great pressure society now places upon women to undergo vaginal delivery, and preferably natural childbirth.

''Some of us apologize for a surgery that may have saved the baby's life and ours as well,'' she says.

A Caesarean Section veteran herself, she writes that few in her age group have escaped the Yuppie mandate to always be success-oriented -- even when it comes to getting an A-plus in childbirth.

The book has important information from birthing to every-day coping with motherhood, and how-to's on diet and exercise.

''Though breast-feeding is a good experience for both the mother and child, there is no reason why bottle feeding can't be, too. Millions of happy, healthy babies have been raised on the bottle. When you can't or don't want to breast-feed, the danger lies not in the bottle but in the possibility that you might communicate any guilt or frustration you feel to your baby.''

The guilt over these and other things makes women apologize to the world for decisions that should always be personal, always.

I hope the '90s will be a time when we learn to kick the habit of blaming ourselves, and always having to explain our decisions.